1. Field of the Invention
The present invention relates to a pressurized fluidized bed combustion combined cycle power plant and method of operating the same; more particularly to a combined cycle power plant comprising a pressurized fluidized bed combustion boiler combustion boiler for burning coal and producing steam,, a gas turbine, a compressor and a generator, with the steam being usable to drive a steam turbine and with means being provided for maintaining a stable fluidization in the pressurized fluidized bed combustion boiler even at partial load.
2. Description of the Prior Art
A known combined plant includes a gas turbine, a steam turbine and a waste heat recovery boiler arrangement so that the steam turbine may be driven with the steam produced in the waste heat recovery boiler by the exhaust gases from the gas turbine. A combined plant of the aforementioned type using a pressurized fluidized bed combustion boiler as a combustor for the gas turbine is generally called a "pressurized fluidized bed combustion combined cycle power plant".
Most known pressurized fluidized bed combustion combined cycle power plants are constructed by employing a double-spindle type gas turbine unit as schematically illustrated in FIG. 2. At full-load operation of this plant, the combustion air boosted by the gas turbine compressor, is completely introduced into the pressurized fluidized bed combustion boiler, in which a fuel or coal is poured and burned to operate the gas turbine. On the other hand, the steam is produced by recovering the heat in the pressurized fluidized bed combustion boiler and the waste heat recovery boiler of the gas turbine and is led into the steam turbine to operate the steam turbine. However, at partial load operation the combustion air flow rate and the pressure in the pressurized fluidized bed combustion boiler are varied for the partial load operation of the plant by changing the speed of the gas turbine compressor.
If, however, a single-spindle gas turbine unit employed is the speeds of both the compressor and the turbine are always the same, because the generator, driven by the same spindle, must rotate at constant speed. As a result, full-load operation is performed as with the double-spindle gas turbine. For a partial load operation, however, the discharge air flow rate of the gas turbine compressor is not changed from that in the full-load operation, nor is the pressure in the pressurized fluidized bed combustion boiler changed, so that, in principle, partial load operation of the pressurized fluidized bed combustion combined cycle power plant is impossible. The present invention overcomes this problem, as described below.
In, for example, JP-A-55-164732, a power plant having a double-spindle gas turbine is disclosed wherein, in order to limit the temperature of the combustion gas reaching the turbine, a bypass line, controlled by a valve, is disposed between the compressor outlet line and the turbine inlet line.
Other prior art documents showing a control element or division in the compressor outlet line include JP-A-63-75406 wherein the line is divided, to provide two controlled air flows to the pressurized fluidized bed combustor, in order to improve combustion efficiency by increasing the temperature at the top of the combustor. Similarly in JP-A-57-212309, the compressed air line divides, in order to supply air to a second burner arranged in the combustion gas line to the gas turbine. In U.S. Pat. No. 4,838,209, valves are provided in the compressed air line, the gas turbine inlet line and a bypass line connecting these two lines, with the two lines being used for emergency shut-down. In U.S. Pat. No. 4,852,345 branch line from the main compressed air line includes a valve and supplies air to a distributor for cooling fluidized bed material in an ash chamber. After cooling the bed material, this air is used in the combustion.
A general review of the development of pressurized fluidized bed combustion and its application in combined cycle power plants is given by Pillai in pages 555-593 of "Electricity: efficient end-use and new generation technologies and their planning implications", published by Lund University Press, Sweden and Chartwell-Bratt, England.